Method of heat treating nickel base alloy articles up to 20 mils in thickness



Se t. 13, 1966 J. SYMONDS 3,

METHOD OF HEAT TREATING NICKEL BASE ALLOY ARTICLES UP T0 20 MILS IN THICKNESS Filed Dec. 9, 196.3

JOHN SYMONDS BY )fim W AGENT United States Patent 3 272,666 METHOD OF HEAT TREATIN G NICKEL BASE ALLOY ARTICLES UP TO 20 MILS EN THllCKNESS John Symonds, Annapolis, Md., assignor to E. I. du Pont de Nemours and Company, Wilmington, DeL, a corporation of Delaware Filed Dec. 9, 1963, Ser. No. 329,128 3 Claims. (Cl. 148-13) This invention relates to improvements in processing nickel base alloys and more particularly to heat treating nickel base alloys to improve their ductility.

The alloys which can be processed by this invention contain nickel as the major constituent and also contain as essential elements from 0.04 to 0.15% of carbon, 5 to 25% chromium and from 1 to of at least one member selected from the group Off titanium and aluminum. Alloys of the above type are very useful because of their high strength and resistance to oxidation at elevated temperatures up to about 1400 F. Many of the most advantageous uses are in relatively thin cross-section forms such as sheets and tubing, said forms being obtained by well-known col d working operations such as rolling and drawing. In order to perform these cold working operations, the alloy must be heat treated to give it adequate ductility; but since the property of ductility is associated with relatively low strength, the heat treated alloy must undergo subsequent treatment to develop the ultimate strength desired.

The prior art shows various methods of heat treating nickel base alloys to improve their ductility for cold working operations. One such method, described in Metal Progress, November 1960, by L. A. Weisenberg and R. J. Morris, shows the heat treating of a nickel base alloy, Rene 41, Trademark Registration Number 678,- 743 by the General Electric Co. (a typical analysis 1s, by weight, 0.08% carbon, 0.50% iron, 19.00% chromium, 11.00% cobalt, 9.75% molybdenum, 3.15% titanium, 1.50% aluminum, 0.006% boron, and the remainder nickel). The reference process comprises heating the alloy in the temperature range of from 1950 to 2150 F. for 30 minutes or longer, followed by quenching and then aging at from 1400 to 1600 F. for several hours to develop the high strength properties. However, when the high temperature solution heat treatment of the reference process is carried out in the presence of air, surface oxidation becomes extensive and vigorous descaling and pickling operations have to be performed. Such operations are very troublesome on a commercial scale where long coils of alloy strips are to be treated. Also, the oxidation which occurs during the relatively long time period of the heat treatment depletes certain minor components of the alloy with the resulting adverse effeet on the alloy properties.

An object of the present invention is to provide a new solution heat treatment for processing nickel base alloys to improve their ductility. A further object is to provide a continuous process of solution heat treating of nickel base alloys in sheet form. A still further object is to minimize surface oxidation in the solution heat treatment. Other objects will be apparent from the description of the invention given hereinafter.

The above objects are accomplished according to the present invention by a new solution heat treatment which utilizes substantially faster time cycles and slightly higher temperatures than used in the prior art and is performed in the relative absence of moisture. This invention is an improvement in the initial step of solution heat treatment of nickel base alloys in the overall process which involves the additional steps of quneching and age hardening.

3,272,666 Patented Sept. 13, 1966 Due to the rapid heating rate needed for the heat treatment, the new process can be used on alloy strips having thicknesses of not over 20 mols. The process broadly comprises heating the alloy for a period of not over 70 seconds nor less than '15 seconds to a temperature in the range of from 1975 to 2200 F. and for a sufficient time to effect solution of the gamma prime phase, i.e., Ni (Ti, Al). At the lower temperatures and shorter times, the heat treatment may be carried out in air but is preferably performed in an inert or hydrogen atmosphere or in a vacuum. By using a vacuum, hydrogen or an inert gas such as argon as the contacting atmosphere, the surface of the alloy remains clear and mirror-like surfaces are obtained. Atmospheres used should have dew points in the range of from 60 to F. After rapid cooling, the alloy at this stage is cold workable and hence may be rolled, drawn, or otherwise fabricated and then age hardened by prior art methods to give a super alloy of full strength and excellent lmetallographic properties.

The accompanying drawing further illustrates the process of this invention. The figure shows the time in seconds plotted against the temperature in degrees Fahrenheit on a semilog plot. The area defined by ABDC depicts the range of holding time at each of the maximum temperatures selected between 1975 and 2200 F for treating the alloy wherein A corresponds to retaining the alloy for 15 seconds at 2200 F., B to 21 seconds at 2200 F., D to 70 seconds at 1975 F. and C to 46 seconds at 1975 F. As can be readily seen from the figure, the operative condition of time needed to effect solution of the gamma prime phase is a function of the temperature at which the alloy is retained. For example, at 2000 F to effect solution of the gamma prime phase, the alloy can have a permissible retention time varying from 40.5 seconds to 61 seconds.

The offset area A'B'DC shows the total time during which the alloy is above 1975 F. prior to beginning the cooling step. This introduces the critical relationship of the necessary heating rate for the process. For example, when 2200 F. is to be the maximum temperature and the alloy is to be retained at that temperature for 21 seconds, the heating rate must be such that the alloy temperature will be raised from 1975 to 2200 F. within 4 seconds. This rapid heat up is preferable since it permits practical continuous processing of long rolls. Similarly, for processing the alloy for 15 seconds at 2200 F., the heat up time from 1975 F. may range from 2 to 10 seconds.

Thus, the solution heat treatment of this invention comprises two critical time-temperature relationships. First, the maximum temperature at which the alloy is to be held defines the permissible range of the retention time. This relationship has been found necessary to insure optimum results in effecting solution of the gamma prime phase. Secondly, after the maximum temperature and retention time of the alloy have been selected, the permissible range of the heating rate is correspondingly established. Maintaining the heating rate within the critical limits enables the heat treatment to be carried out with a minimal occurrence of surface oxidation as well as making continuous processing operations practica As an illustration of the effect of heat treatment outside this critical range, it was found that heating for 34 seconds at 2200 F. resulted in excessive grain growth, solution of the desired carbide phase and weakening of the metal to the extent that, even after final aging, the yield strength was below 100,000 p.s.i.

Following the solution heat treatment, the alloy is rapidly cooled, preferably to about 1300" F. in not more than 20 seconds. At this stage, the alloy is ductile and has relatively low strength as will be shown by the examples. According to the presently accepted explanation given in the Metal Progress article by Weisenberg and Morris, this rapid cooling rate is necessary to prevent premature precipitation of the gamma prime phase. According to well-known techniques, the alloy can now be drawn or rolled to a thinner gauge and thereafter aged at about 1400 F. for several hours to properly precipitate the gamma prime phase and to develop the necessary ultimate high strength.

For a clearer understanding of the invention, the following specific examples are given. These examples are intended to be merely illustrative of the invention and not in limitation thereof.

Example I treatment. Its tensile properties were:

Yield strength, p.s.i 90,000 Ultimate tensile strength, p.s.i 159,000 Percent elongation 32 The second portion was aged at 1400 F. for 12 /2 hours in vaccum and was found to have the following properties:

Room Temp. 1,400 F.

YS, p.s.i 191,000 110,000 UTS, p.s.i 222,000 115, 000 Percent Elongation 11 12 The surfaces were still good and the gauge uniform.

Example 11 The method of Example I was repeated using the same type of alloy except that the solution heat treatment was carried out at 2200 F. for 18 seconds with the total time above 1975 F. estimated at about 23 seconds. The properties obtained were as follows:

As Quenehed Aged 58, 000 175,000 128, 000 222, 000 Percent Elonga 34 12% Example III A 24 inch wide coil of commercial Ren 41 alloy strip .010 inch thick by 200 ft. long was solution treated in a strip furnace under vacuum. The hot zone of the furnace was 30 inches long and was maintained at 2065 F. F. while the strip was being passed through this zone at 40 inches/minute. It was estimated by means of thermocouples in contact with the strip, that the total time above 1975 F. prior to the start of cooling was 42 seconds and the time at 2065 F. L5 F. was 37 seconds. The strip was cooled rapidly by passing through a close clearance water cooled metal enclosure immediately adjacent to the heating zone. Properties of the alloy at this stage were:

YS, p.s.i 86,000 UTS, p.s.i 160,000 Percent elongation 32 The strip was then subjected to aging in air at 1400 F. for 16 hours. Its properties were then:

At Room At 1,400" F.

Temp.

The high ductility in combination with such high tensile strengths which was obtained was unusually good in comparison with prior art products.

Example IV Samples of Ren 41 were placed in a furnace in an argon atmosphere at the following temperatures and times:

Time at Tem- Time at or Temperature, perature, above 1,975 F. F., seconds seconds before start of cooling, seconds After the high temperature treatment, the samples were examined by metallographic and electron microscope means. This examination showed that the desired M C network structure was not dissolved and that the desired fine grain structure was present.

'In the foregoing examples, the properties following the solution heat treatment, yield strengths below 100,000 p.s.i. and high ductility indicated by the more than 30% elongation, are such that easy forming is obtained. For example, the solution treated and quenched strip of Example III may be further reduced to form one of the elements for a honeycomb structure after which said structure may be aged to develop the high strength properties needed for such structures.

Alloys which can be processed by the solution heat treatment of this invention are numerous and can generically be described as nickel base alloys which have high strength and resistance to oxidation at elevated temperatures. These can be more specifically characterized as nickel base alloys which comprise, by weight, nickel within the range of 48 to and contain as essential elements from 5 to 25% chromium, 0.04 to 0.15% carbon and from 1 to 10% of at least one member selected from titanium and aluminum and may additionally contain up to 25% molybdenum and smaller amounts of tungsten, zirconium, silicon, manganese, iron, boron and columbium plus incidental impurities.

Within the scope of this invention, the nickel alloys may be heated by immersion, in suitable baths, such as a molten salt bath for the required time after which said nickel alloys can be transferred to a cooling medium such as water or brine.

The discovery of this short time, high temperature treatment has made practical and continuous treatment of long strips through the heating and quenching steps. Because of the relatively short retention times necessary to effect solution of the gamma prime phase, the length of the heating Zone is but a matter of inches at reasonable processing speed. Accordingly, it is preferred to maintain the maximum temperature within the range of from 2150 to 2200 F. Commercially, this means that the size of the furnace or other heating device is economically feasible and the speed of travel is fast enough so that the metal moves out of the heating zone into the cooling zone in the required short time for the cooling step.

This sequence was not practical for previous heating schedules. The improvement of this invention has therefore made possible the processing of long coils of sheet alloys with resulting excellent properties not heretofore possible. To further shorten the length of the heating zone as well as to minimize oxidation, it is desirable to heat the alloy in the solution heat treatment step at as rapid a rate as possible, preferably at at least an average of 100 F. per second within the range of from about 1400 F. to the maximum treatment temperature.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.

The invention claimed is:

1. A method of heat treating a nickel base alloy object of up to 20 mils in thickness containing as essential elements, by weight percent, from 0.04 to 0.15 carbon, 5 to 25 chomium and from 1 to of at least one element selected from the group consisting of aluminum and titanium, which comprises heating said nickel base alloy object to a maximum temperature of from 1975 F. to 2200 F., retaining said nickel base alloy object at said maximum temperature for a period of time such that when the retaining time is plotted against said maximum temperature, the resultant point falls within the area ABDC as set forth in the accompanying drawing, and maintaining the heating of said nickel base alloy object such that when the retaining time plus the heating time from 1975 F. to the maximum temperature is plotted against said maximum temperature, the resultant point falls within the area AB'DC as set forth in the accompanying drawing.

2. A method of heat treating a nickel base alloy object of up to 20 mils in thickness containing as essential elements, by weight percent, from 0.04 to 0.15 carbon, 5 to chromium and from 1 to 10 of at least one element selected from the group consisting of titanium and aluminum, which comprises heating said nickel base alloy object at a rate such that the temperature of said nickel base alloy object is raised from 1975 F. to 2150 F. in not more than 3 seconds, retaining said nickel base alloy object at 2150 F. for 25 seconds and thereafter rapidly cooling said nickel base alloy object.

3. A method of heat treating a nickel base alloy object of up to 20 mils in thickness containing as essential elements, by weight percent, from 0.04 to 0.15 carbon, 5 to 25 chromium and from 1 to 10 of at least one element selected from the group consisting of titanium and aluminum, which comprises heating said nickel base alloy object at a rate such that the temperature of said nickel base alloy object is raised from 1975 F. to 2200 F. in not more than 5 seconds, retaining said nickel base alloy object at 2200 F, for 18 seconds and thereafter rapidly cooling said nickel base alloy object.

References Cited by the Examiner UNITED STATES PATENTS 3,028,268 4/1962 Tisinai et a1. 148-13 3,145,124 8/1964 Hignett et a1 148162 3,146,136 8/1965 Bird et a1. 148162 3,207,599 9/1965 Franklin et a1 148-162 DAVID L. RECK, Primary Examiner.

RICHARD O. DEAN, Assistant Examiner. 

1. A METHOD OF HEAT TREATING A NICKEL BASE ALLOY OBJECT OF UP TO 20 MILS IN THICKNESS CONTAINING AS ESSENTIAL ELEMENTS, BY WEIGHT PERCENT, FROM 0.04 TO 0.15 CARBON, 5 TO 25 CHROMIUM AND FROM 1 TO 10 OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND TITANIUM, WHICH COMPRISES HEATING SAID NICKEL BASE ALLOY OBJECT TO A MAXIMUM TEMPERATURE OF FROM 1975* F. TO 22000*F., RETAINING SAID NICKEL BASE ALLOY OBJECT AT SAID MAXIMUM TEMPERATURE FOR A PERIOD OF TIME SUCH THAT WHEN THE RETAINING TIME IS PLOTTED AGAINST SAID MAXIMUM TEMPERATURE, THE RESULTANT POINT FALLS WITHIN THE AREA ABDC AS SET FORTH IN THE ACCOMPANYING DRAWING, AND MAINTAINING THE HEATING OF SAID NICKEL BASE ALLOY OBJECT SUCH THAT WHEN THE RETAINING TIME PLUS THE HEATING TIME 